In some configurations, a raid6 resync can be limited by CPU speed(Calculating P and Q and moving data) rather than by device speed.In these cases there is nothing to be gained byt serialising resyncof arrays that share a device, and doing the resync in parallel canprovide benefit.So add a sysfs tunable to flag an array as being allowed toresync in parallel with other arrays that use (a different part of)the same device.